This invention relates to molding compositions and more particularly to improved compositions and a process for preparing same wherein such compositions are especially adapted for shaping into products such as packaging materials wherein free styrene(S) monomer therein is reduced.
Thermoplastic polymer compositions containing polymerized S can be shaped into a wide variety of useful articles by conventional techniques such as casting, extrusion, milling, molding, drawing, blowing, etc. Applications for such shaped articles are widespread and include structural units where properties such as low thermal deformation, impact resistance (when a rubber component is present) and high gloss are required. When acrylonitrile is present with styrene in a copolymer at a concentration of at least about 50 weight percent, the compositions uniquely exhibit excellent solvent resistance and low permeability to liquids and gases which make them especially useful as a lightweight substitute for glass in packaging and particularly in the manufacture of bottles, film, sheet, tubs, cups, trays and other containers for liquids and solids.
In manufacturing such polymer compositions, it is well known that free, unconverted S monomer remains absorbed within the polymer particles when polymerization is not 100% complete and which is therefore present in products formed therefrom. Recently government regulatory agencies are moving toward establishing maximum permissible levels of various monomers in the environment on grounds that excess levels may constitute a health hazard, and particularly regulations have been applied to packaging materials intended for contact with environmentally sensitive products such as food, beverages, pharmaceuticals, cosmetics and the like for which application styrenic copolymers are especially suited.
Even though possible to reduce residual styrene monomer (RST) in styrenic polymers before melt processing via stripping, it has been discovered that free S monomer is thermally regenerated due to polymer unzipping during melting at standard melt processing times and temperatures. Depending on the level of S present in the polymer before melting, such an increase could be sufficient to (i) be extractable from shaped packaging materials in contact with food simulating solvents and/or (ii) be released into the atmosphere around melt processing equipment. Accordingly, reduction of S monomer in the raw polymer could be ineffectual in controlling S in shaped products formed in an operation involving melt processing.